The use of acidic materials as catalysts in chemical reactions is well known in the prior art. Illustrative of chemical conversions which are acid-catalyzed are the production of alcohols by the hydration of olefins, the production of esters by the interaction of carboxylic acids with olefins, the production of ethers by dehydration of alcohols or by the reaction of alcohols with olefins, the alkylation of aromatic hydrocarbons, the cracking of gas oil, the polymerization of olefins, the isomerization of hydrocarbons, and the like.
Among the catalysts employed for acid catalysis are mineral acids, heteropoly acids and ion exchange resins. Catalysts which are known for the esterification of olefins with carboxylic acids include silica-alumina and other various mixtures of refractory metal oxides. The activity of metal oxide catalysts usually is promoted by acid pretreatment of the catalysts.
U.S. Pat. No. 2,174,985 discloses a process for the esterification of olefins in vapor phase in the presence of a heteropoly acid catalyst such as phosphotungstic acid supported on activated alumina.
U.S. Pat. No. 3,053,887 discloses a method for esterification of isoolefins at a temperature below about 5.degree. C. in the presence of a catalyst consisting of a sulfonated copolymer of styrene crosslinked with divinylbenzene.
U.S. Pat. No. 3,492,341 describes a process for the esterification of monoolefins with carboxylic acids at 100.degree.-800.degree. C. in the presence of a mordenite type crystalline aluminosilicate catalyst predominantly in the hydrogen form.
U.S. Pat. No. 3,922,294 describes a gas/liquid phase process for the manufacture of isopropyl esters which involves passing propylene and a carboxylic acid continuously in parallel flow over an acid ion exchanger of the sulfonic acid type. The service life of the catalyst is improved by the addition of a strong mineral acid to the liquid phase.
U.S. Pat. No. 3,920,582 and 3,932,306 describe two related types of solid catalysts for heterogeneous reactions such as hydration of olefins, esterification of olefins, and the like. One type of catalyst is a composite consisting of a solid carrier and an acidic compound containing at least two sulfonic groups per molecule. The other type of catalyst is a composite consisting of a solid carrier and carboxymethane sulfonic acid.
U.S. Pat. No. 4,011,272 describes a process for producing tertiary butyl alcohol which involves reacting isobutylene with an aqueous organic acid solution in the presence of an acidic ion exchange resin.
There has been continuing development effort to overcome the many difficulties characteristic of acid catalyzed synthesis methods. It is well known that acid catalysts are highly corrosive to processing equipment, and that the cumulation of metal ions resulting from the corrosion has a deactivating effect on the catalyst. In reactions involving olefins there is the problem of undesirable olefin polymerization side reactions which lower the overall efficiency of the processes, and which have the additional disadvantages of contaminating and deactivating the acid catalysts and complicating reaction product recovery procedures.
There remains a need for improved methods and catalysts for overcoming the difficulties and inefficiencies of acid catalyzed processes.
Accordingly, it is an object of this invention to provide a novel strongly acidic solid catalyst which exhibits long term stability and reactivity under acid catalyzed heterogeneous reaction conditions.
It is another object of this invention to provide a strongly acidic solid catalyst which is highly selective for the production of esters by the reaction of olefins with carboxylic acids.
It is a further object of this invention to provide an improved process for the esterification of an olefin with a carboxylic acid, wherein the conversion selectivity of the carboxylic acid to ester product is substantially quantitative under optimal conditions.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.